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Science with the Hubble Space Telescope -- II
Book Editors: P. Benvenuti, F. D. Macchetto, and E. J. Schreier
Electronic Editor: H. Payne

Distant Galaxies

Malcolm S. Longair
Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, England.

 

Abstract:

A wide variety of recent HST observations of distant galaxies is reviewed. Redshifting the images of nearby galaxies indicates that they are difficult to observe with the HST at redshifts greater than 1, unless they were more luminous or had greater surface brightnesses in the past. A number of observations suggest that there is an increase in the fraction of irregular and high surface brightness galaxies with increasing redshift. At redshifts greater than 2, there is strong evidence that many galaxies underwent bursts of continued star formation and that, at the largest redshifts, , the co-moving space density of star-forming galaxies is approximately half of that of luminous galaxies () at the present day. Radio galaxies at redshifts have high optical surface brightness emission regions stimulated by the passage of the radio jets responsible for powering their large-scale radio structures.

Keywords: galaxies, cosmology, galactic evolution, radio galaxies

Introduction

It is impossible to do justice in a modest space to the vast amount of new information which has resulted from Hubble Space Telescope observations of distant galaxies. I will touch briefly on the following areas which will be dealt with in much more detail in the contributed papers and in the posters:

  1. The observability of galaxies at large redshifts;
  2. Galaxy counts and the origin of the excess of blue galaxies;
  3. Galaxies in the redshift interval 1 < z < 2;
  4. Galaxies at very large redshifts;
  5. Gravitational lenses and galaxies at large redshifts;
  6. Radio galaxies at redshifts .

The Observability of Distant Galaxies

It is important to appreciate exactly what it is that we observe when optical HST observations extend to redshifts . In fact, the predictions are not so different from those presented by Gunn (1979). For example, Figure 1(a) shows the nearby giant spiral galaxy NGC 5364 at a redshift of 0.004137 and Figure 1(b) the same galaxy as it would appear in an 8000 second observation through the I filter, F814W, if it had redshift (Abraham et al. 1996). It can be seen that the spiral structure is still just detectable but, at greater redshifts, it would be impossible to distinguish details of its morphological structure.

 
Figure: (a) NGC 5364 as observed from the ground; (b) NCC 5364 as it would be observed at a redshift by the WFPC2 in an 8000 second integration through the I filter, F814W (Abraham et al. 1996).

The above simulation was made using estimates of the K-corrections for spiral galaxies. Giavalisco et al. (1996) have described an improved procedure using Astro-1 Shuttle observations of nearby galaxies in ultraviolet wavebands at 152 nm and 249 nm. The galaxies M32, M31, M33, M74, M77, M81 and M82 were observed in these wavebands by the Ultraviolet Imaging Telescope of the Astro-1 mission and Giavalisco and his colleagues have used these to determine the morphologies and apparent magnitudes these galaxies would have if observed by the WFPC2 in 4.3 hour integrations. Some of their results are summarised in Table 1.

The figures enclosed in square brackets in Table 1 indicate that, if the galaxies were located at these redshifts, they would not be observable by the HST because they have too low surface brightnesses. NGC 1068 is a special case since it possesses a Seyfert nucleus and so can be observed even at the very large redshift z = 3, but the host galaxy would be undetectable. The two other galaxies which are just detectable at redshift are observed with too low signal-to-noise ratio for them to be classified morphologically. These computations make the important point that, if galaxies are to be classified successfully at redshifts z > 1 by the HST, they must either have greater surface brightnesses or luminosities than the typical galaxies we observe nearby. We will find that there are good observational reasons why this is, in fact, the case. It is also clear that it is possible to make excellent progress in the morphological classification of galaxies out to redshifts of about 0.5, and greater for the more luminous galaxies.

 
Table: Local galaxies observed by the Ultraviolet Imaging Telescope of the Astro-1 Shuttle Mission. The table shows the expected magnitudes of the galaxies if they were observed at redshifts and by the HST. Square brackets indicate that the galaxies would be of too low surface brightness to be detected (Giavalisco et al. 1996).

Morphological Studies of Galaxies at Moderate Redshifts

Several studies of the morphologies of faint galaxies have been reported (Glazebrook et al. 1995a, Abraham et al. 1996, Driver et al. 1995,1996). Different approaches have been taken to the assignment of morphological types. The validation of computer algorithms to assign types is of importance in order to determine the morphological mix of the faintest galaxy samples. In the approach taken by Abraham et al. (1996), morphological types have been assigned by galaxy experts and these have been compared with `objective' machine classifications based upon two parameters which can be conveniently defined for each galaxy image. These are the compactness of the brightness distribution within the image of the galaxy, C, and its asymmetry, A, determined by rotating the galaxy image through 180 and defining objectively its degree of asymmetry. These parameters have been determined for a sample of galaxies from the HST Medium Deep Survey, which has been carried out in parallel mode for a number of random fields at high galactic latitudes. Typically, the exposures were for 5000 seconds and 507 objects were classified with I<22. The A-C diagram is shown in Figure 2, in which the morphological types have been assigned by van den Bergh according to a scheme in which compact, elliptical and S0 galaxies are denoted by ellipses, spiral galaxies of all types by spirals, and irregulars, peculiar and merging systems by crosses. The diagonal lines on Figure 2 show the boundaries within which it is expected that the three different morphological classes are found, on the basis of redshifting examples of Hubble types to faint apparent magnitudes. According to Abraham et al., the machine assignments agree very well with the `expert' classifications down to apparent magnitude I = 21, but there is more scatter in the assignments in the faintest magnitude interval, 21 < I < 22.

 
Figure: The distribution of galaxies on the central concentration, C, versus asymmetry, A, plane. The morphological assignments by van den Bergh are shown according to a scheme in which compact galaxies, ellipticals and S0 galaxies are denoted by ellipses, spiral galaxies of all types are designated by spirals, and irregulars, peculiar and merging systems are indicated by crosses. Representative error bars for different regions of the diagram are shown. The regions of the plane corresponding to the three morphological categories, as derived from an artificially redshifted sample of local galaxies, are also shown (Abraham et al. 1996.)

These procedures are of considerable importance for understanding the excess counts of faint blue galaxies observed in deep galaxy surveys. Figure 3 shows the counts of galaxies of different morphological types. The solid lines show the expected counts for uniform world models with and it can be observed that the E/S0 and spiral samples follow closely these expectations. The important result is that the excess blue galaxies are associated with the galaxies which are classed as Irregulars/Mergers.

 
Figure: The counts of faint galaxies of different morphological types. The counts as determined by van den Bergh, Ellis and according to automatic classification procedures for the same sample of galaxies are indicated by different symbols. The solid lines show the expected galaxy counts for a uniform world model with no evolution. The pronounced excess of irregular/merging galaxies is apparent (Abraham et al. 1996).

A similar result has been found by Driver et al. (1995) in their analysis of a single ultra-deep WFPC2 deep field which was observed for 5.7 hours in each of the V (F606W) and I (F814W) wavebands. These samples extend somewhat deeper than the Medium Deep Survey, the limiting apparent magnitudes being , corresponding roughly to . In their analysis, the classification of the images was carried out by fitting surface brightness profiles to the galaxy images, as well as by visual inspection. The ellipticals are those for which the surface brightness distributions follow de Vaucouleurs' law and the spirals those with exponential light profiles. Those with light profiles which follow neither of these distributions are classified as irregulars. They find exactly the same result as in the Medium Deep Survey, but now the counts extend to significantly fainter apparent magnitudes.

In an intriguing study, Neuschaefer et al. (1996) analysed the spatial distribution of galaxies in 28 fields from the Medium Deep Survey and in the Groth-Westphal survey, which consisted of 28 contiguous fields at high galactic latitudes. Many close pairs and apparently interacting systems of galaxies were noted among the blue population. One of the most striking results of their analysis is the determination of the two-point correlation function for galaxies down to the very smallest angular separations. Despite the fact that there are apparently many associations of faint galaxies in the samples, the two-point angular correlation functions seem to follow the standard result down to angular scales as small as 2 arcsec, with only a small excess of assocations at small angular radii. Thus, although it might have been expected that there would be a large excess of nearest neighbours if the blue galaxies were due to interacting or coalescing systems, it seems that the very modest excess does not greatly exceed what might be expected from the standard two-point correlation function.

There has been considerable debate about the nature of the excess faint blue galaxy population. It had been expected that there might well be more blue galaxies at faint apparent magnitudes because even passively evolving models of galaxies suggest that the old stellar population should be brighter in the past and there should be more star formation activity. One of the surprises of the first redshift surveys, which extended to apparent magnitudes at which the blue excess was observed, was that the mean redshift of the galaxies at the faintest magnitudes did not increase any more rapidly than would have been expected if the galaxy population had remained unchanged with cosmic epoch (Glazebrook et al. 1995b). This result was interpreted as indicating that the excess blue galaxies were associated with a population of dwarf galaxies. There must, however, be more to the story than this.

Galaxies at Redshifts

It is now becoming possible to study fainter samples of galaxies and three examples indicate what is now becoming the state of the art. At this meeting, Schade reported the most recent results of the Canada-France Redshift Survey (Le Fèvre et al. 1995). This magnitude-limited complete survey contains 943 objects with magnitudes . The objects for which redshifts have been measured extend to redshifts . HST images have been secured for 32 randomly selected galaxies with redshifts and these display the normal range of morphological types. There are, however, important differences. The mean rest frame surface brightnesses of the late type galaxies are about 1.2 magnitudes greater than those of nearby galaxies. Some degree of peculiarity/asymmetry is observed in 30% of the objects, and 13% show clear signs of mergers or interactions. There are compact blue components in 30% of the galaxies and these occur predominantly in the peculiar systems, but a few of them are also present in normals systems. According to Schade et al. (1995), these galaxies are predominantly the cause of the excess of faint blue galaxies and the numbers are consistent with those found in the morphological surveys described in Section 3.

The second example concerns the recent results of Cowie et al. (1995) on the redshift distribution of faint galaxies. The galaxies were selected from very deep surveys in small areas of sky and consisted of all the objects in these areas which satisfied the magnitude selection criteria K < 20, and . Spectra for these galaxies have been obtained with the Keck 10-metre telescope and the programme has been remarkably successful in discovering large redshift galaxies. There were 367 objects which satisfied the selection criteria and, among them, 91 are already known to be galaxies with redshifts and 40 with redshifts z > 1. The reason for their success is immediately apparent from the typical spectra of the galaxies for which spectra have been obtained (Figure 4). The large redshift galaxies have strong emission lines, characteristic of regions of star formation. According to their analysis, these are luminous galaxies with absolute B magnitudes, . The luminosities of the large redshift galaxies in the [OII] line are typically at least an order of magnitude greater than those of a reference sample at redshifts . They infer that these are star-forming massive galaxies at redshifts and the rates at which the stars are being formed can account for between about 5 and 20% of their present stellar populations.

 
Figure: The average rest-wavelength spectra of the objects in the deep survey field SSA13. It is apparent that they are strong emission line objects with only weak absorption lines in the ultraviolet region of the spectrum (Cowie et al. 1995).

Deep HST images have been obtained for nine of the galaxies at redshifts z > 1 in the I waveband. As they note, the galaxies have `strikingly unusual morphologies, often consisting of chains or structures of compact blobs, suggesting that they are generally not dominated by uniformly distributed star formation'. These results suggest that among the faint blue galaxies there is a population of distant star forming galaxies in addition to objects at smaller redshifts.

Similar preliminary results were reported by Koo at this meeting from a deep survey undertaken with the Keck 10-m telescope in the HST Groth strip. The sample extends to I = 24 and in this area they already have redshifts for 35 galaxies in the range , with a mean redshift of about 0.8, significantly greater than that of the Canada-France Redshift Survey. Again, the sample contains a large number of unusual galaxies including objects with multiple knots and the types of chain galaxy reported by Cowie et al. (1995). In addition, they have found nine red galaxies which are as red as elliptical galaxies are at the present epoch. The inference is that these galaxies have already had time to form old stellar populations and so must have undergone their last major burst of star formation at redshifts z > 2.

Another beautiful example of the importance of mergers and interactions between galaxies is provided by the very deep radio-optical survey of Windhorst et al. (1995). They conducted a very deep VLA survey of a small area of sky which reached a flux density limit of about 1 Jy. The corresponding deep optical survey carried out by the HST extends to . It turns out that 60% of the microjansky radio sources are identified in this survey, many of them being associated with pairs or groups of galaxies. These are such intrinsically weak radio sources that they are not much more luminous as radio sources than normal galaxies. It is natural to associate them with the types of interacting and starburst galaxies which have been detected in the IRAS survey, but now these objects typically lie at redshifts .

Galaxies at Large Redshifts

I will discuss two examples of the study of galaxies at very large redshifts. The first of these results from the study by Windhorst & Keel (1995) of what they refer to as a young `elliptical' radio galaxy at a redshift . Although a radio galaxy, and so possibly unusual in it characteristics, it is a relatively modest radio emitter, about 100 times less luminous radio-wise than the objects I discuss in Section 7. In their paper, they derive a surface brightness profile for the galaxy which more or less follows a de Vaucouleurs law, and this is the basis of their claim that it is an elliptical galaxy. At this meeting, Pascarelle described new observations of the field of this radio galaxy which, by great good fortune, lies at such a redshift that the Lyman- line is redshifted into the narrow F410W filter. They find evidence for 18 Lyman- objects at this redshift, all of them with luminosities between about 0.1 and 1 . All of these objects seem to be compact and again the sum of their brightness distributions seems to follow the law. They suggest that this is evidence for the early formation of the bulges of galaxies.

 
Figure: (a). The spectrum of a starburst of duration 12Gyr as observed at different ages (White 1989, from computations by G. Bruzual). (b) Illustrating how three colour photometry in the U, G and R wavebands can isolate star-forming galaxies at large redshifts. The dashed line shows the spectrum of a star-forming galaxy at a redshift z = 3 (Macchetto & Giavalisco 1995).

The largest redshift systems which have been identified as young star-forming galaxies have been discovered by searching for the redshifted Lyman limit by multi-colour photometry. The technique is similar to that described by Lilly & Cowie (1987) and refined for the detection of `Lyman-limit galaxies' by Steidel & Hamilton (1992,1993). The predicted spectrum of a starburst galaxy is illustrated in Figure 5(a) in which it can be seen that, as the starburst ages, the spectrum remains of the same characteristic form, namely, it is flat from the Lyman limit at 91.2 nm to longer wavelengths with an abrupt cutoff at nm. At a redshift z = 3, the Lyman limit is shifted to 400 nm and so the characteristic signature of these objects is that roughly equal intensities are observed in the G and R wavebands but the intensity in the ultraviolet waveband is very low as illustrated in Figure 5(b). The story began with the successful attempt to identify the large redshift absorption systems present in the background quasar QSO 0000-262, which has an emission redshift (Steidel & Hamilton 1992,1993). In this field, Macchetto et al. (1993) identified a `Lyman- radio quiet galaxy' at a redshift . Searches in four other QSO fields are described by Steidel et al. (1995).

Macchetto & Giavalisco (1995) and Steidel et al. (1996) have described further observations of these fields. Macchetto & Giavalisco (1995) described the application of this multi-colour technique to the field containing the galaxy at redshift and several objects with the signature of star-forming galaxies were found. At this meeting, Giavalisco described the exciting result that spectroscopy with the Keck 10-m telescope has confirmed that these objects are indeed galaxies at redshift . These galaxies have been imaged by the WFPC2 and, when the images of the galaxies are summed, they are found to follow the standard de Vaucouleurs dependence of surface brightness upon radius of elliptical galaxies.

 
Figure: The central portion of the field of the cluster Abell 2218 as observed through the F702W filter of the WFPC2. (Kneib et al. 1996).

Steidel et al. (1996) have obtained the spectra of 24 candidate star-forming galaxies selected in both the quasar fields and in random regions of sky and have had great success in measuring redshifts for these with the Keck 10-m telescope. Seventeen of the objects have redshifts in the interval . They find the important result that the co-moving space density of these star-forming galaxies in the redshift interval is about half that of luminous galaxies with at the present epoch. The inferred velocity dispersions within the galaxies suggest that they are indeed massive galaxies. The star formation rates correspond to about about yr, similar to the star formation rates per galaxy found by Cowie et al. (1995). Steidel et al. infer that they have discovered the formation of the spheroidal components of the progenitors of massive galaxies --- massive galaxy formation was certainly well underway by a redshift of 3.

Gravitational Lensing of Very Distant Galaxies

As was emphasised in Section 2, normal galaxies at large redshifts can only be readily observed if their surface brightnesses are enhanced. One clever way of achieving this is to use the gravitational lensing of rich clusters of galaxies to magnify the flux densities of distant background galaxies. A remarkable example of how this can be done is described by Kneib et al. (1996) who have studied in detail the gravitational lensing of distant galaxies by the mass in the central regions of the cluster Abell 2218. Figure 6 shows their beautiful image of the central regions of the cluster with the remarkable gravitationally lensed images of distant background objects. There are several important features of the lensed background objects which enable the mass distribution in the lens to be determined with considerable precision. In particular, the observation of multiple images of the same background object enable the location of the critical lines and the detailed mass distribution to be determined rather precisely.

Once the mass distribution of the lens has been determined, the images of other distant background objects are expected to be sheared and distorted at different projected distances from the centre of the lens, in ways which depend upon their redshifts. Kneib et al. (1996) describe in detail the procedures involved in making these redshift estimates and also the uncertainties involved, among these being knowledge of the intrinsic shapes of the galaxies. Consequently, only statistical estimates of the redshifts of the galaxies can be made. Using these procedures, Kneib et al. have estimated the redshifts of the faint elongated images in the field of the cluster Abell 2188 with the results shown in Figure 7. It is apparent that this technique provides a means of studying the properties of galaxies at much larger redshifts than would have been possible without the intervention of the gravitational lens. Just after the meeting in Paris, Ellis (1996) reported at the ICGC95 meeting in Pune, India, that redshifts have now been obtained for a number of these faint galaxy images and the spectroscopic redshifts are in remarkable agreement, in general, with the estimates based upon the gravitational shearing of their images, confirming the great potential of this technique for studying galaxy evolution at large redshifts.

 
Figure: The estimated redshifts of the faint elongated objects observed in the field of the cluster Abell 2218. The lines show the expected mean redshift distribution for the lensed objects. (Kneib et al. 1996).

Radio Galaxies at Redshifts

Finally, let me describe some of our recent HST observations of radio galaxies at redshifts . The programme consists of WFPC2 imaging observations of a complete sample of 3CR radio galaxies in the redshift interval . These radio galaxies are of special interest because it is known that they exhibit the strong cosmological evolutionary trends observed in the radio source and quasar populations as a whole. It is also known that, in the majority of cases, the optical structures are aligned with the radio axes of these double radio sources. We have observed all these radio galaxies with the HST in wavebands corresponding more or less to rest-wavelength U and B wavebands. These observations have been supplemented by infrared observations at 2.2 m taken with UKIRT which have angular resolution of about 1 arcsec and by 8.4 GHz VLA observations with angular resolution 0.18 arcsec. The infrared observations provide images of the old stellar populations in these galaxies and they all resemble standard giant elliptical galaxies.

 
Figure: HST and UKIRT images of the radio galaxies 3C 266, 368, 324, 280 and 65 with the VLA radio contours superimposed. The images are drawn to the same physical scale (Best, Longair & Röttgering 1996a).

 
Figure: HST and UKIRT images of the radio galaxies 3C 267, 252 and 356 with the VLA radio contours superimposed. The images are drawn to the same physical scale (Best, Longair & Röttgering 1996a).

The high resolution HST images are dramatically different. Virtually all the images show emission regions aligned with the jets which are assumed to be powering the hot-spots in the outer radio lobes. Perhaps the most striking result is the comparison of the maps of all the radio galaxies in our sample in the redshift interval (Best, Longair & Röttgering 1996a). Since these are all 3CR radio galaxies, the radio sources have the same intrinsic luminosities, implying that the jet luminosities are the same for all of them. Figures 8 and 9 show a montage of these eight radio galaxies in order of increasing separation between the components of the double radio sources. It can be seen that the most remarkable structures are associated with the smaller double radio sources. The optical emission regions associated with the radio galaxies 3C 266, 368, 324 and 280 are all aligned along the axis of the double radio sources. Comparison of the optical and infrared images shows that the optical structures are on more or less the same physical scale as the host galaxy. As the sizes of the double source increase, the optical emission regions become less prominent and, although there is still some alignment with the radio axis, the structures are on a smaller physical scale. According to the theory of double radio sources, the large sources are older than the smaller sources and what is of particular interest in these cases is that, for some of them, synchrotron ages are available. These arguments suggest that the radio sources associated with 3C 266 and 280 are a few million years old (Liu, Pooley & Riley 1992).

These observations suggest that the strongly-aligned optical structures are short-lived phenomena, which are stimulated by passage of the radio jet. As we have pointed out already (Longair, Best & Röttgering 1995), it seems that no single theory of these alignments can account for all the observations. The observation of polarised optical emission from some of these sources suggests that scattering of light from an obscured quasar must play some role in accounting for the emission from these optical structures. It seems, however, that the primary cause of the structures must be the interaction of the radio jet with cool interstellar clouds within the parent galaxy. Exactly how these structures are formed is unclear. One possibility is that the structures are associated with jet-induced star-formation. It would then be possible to account rather naturally for the change in structure with increasing physical size. The lifetime of the newly formed stars and associated HII regions would amount to only about years. After this time, the luminosities of the star-forming regions would decay and the star clusters relax within the potential of the parent giant elliptical galaxy. The polarisation of the light would be attributed to the scattering of the light of an obscured quasar by the dust or gas associated with the star-forming regions. A problem with this picture is that there have so far been no reports of young stars in the spectra of the emission regions. Alternatively, the alignments may be due to the illumination of pre-existing dust and gas clouds by a central obscured quasar. There remains the problem of accounting for the presence of large amounts of cool gas and dust within the body of the parent galaxy.

 
Figure: (a) The HST image of the radio galaxy 3C 34 superimposed upon which are the VLA radio contours of the double radio source structure. (b) and (c) Images of the structure of the optical `jet' which lies along the line from the nucleus of the galaxy to the western hot-spot as observed through the f555W and f785LP filters respectively. (d) J and (e) K images of the galaxy associated with the optical jet observed with UKIRT (Best, Longair & Röttgering 1996b)

One remarkable result, which may be evidence for jet-induced star-formation, has been found in the field of the radio galaxy 3C 34, which has a redshift of 0.69. In this source, the radio galaxy is rather diffuse with no prominent structures such as those found in the smaller double sources, as can be seen in Figure 10. However, along the axis from the nucleus of the radio galaxy to the brightest western hot-spot, there is a remarkable linear feature which passes close to the galaxy labelled (a). These structures are shown in more detail in Figure 10(b-e). The linear feature is bluer than other objects in the field. Our interpretation of this feature is that they may represent the interaction of the radio jet with the interstellar gas in a galaxy which just happens by accident to lie in the path of the jet.

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